NTA.

Regardless of all the rest of the details, anybody who can walk a dog, but does not walk a dog, when the dog needs to be walked... is an asshole.

Because there's a third person here: THE DOG. And it's not okay to mistreat somebody who is unable to care for themselves (in this case, the dog) just because it's technically not your chore.

When you add in all the details -- like the fact that you were sick -- that just makes your mom's behavior even worse. Mature, reasonable people who love each other automatically choose to take on a little extra work if the person they love is sick.

It sounds like maybe she was trying to teach you a lesson of some sort, about how dogs are a full time responsibility no matter what, but it's not fair for her to punish the dog because of her opinion about her own behavior.

When I was a teenager my mom got a dog I actually had not wanted (I knew she wouldn't take care of it -- she was very irresponsible -- and I was sort of burnt out on dogs since she'd neglected the last two as well). Like I predicted, she did not take care of the dog. I guess I could have just let the dog suffer to teach my mom a lesson or something, but I'm not a monster, so of course I took care of it even though I hadn't wanted to take on the care of a dog.

In her case, she didn't even have a good excuse. You did. And you expected that your mom would be a nice person who steps up when you're sick and takes care of an animal who needs to be cared for.

Well, now you know she won't. In the future you'll have to walk the dog even if you're practically on your death bed, since you know she won't. At least you know what the situation is now.